Thick-film pastes containing lead- and tellurium-oxides, and their use in the manufacture of semiconductor devices

ABSTRACT

The present invention provides a thick-film paste for printing the front-side of a solar cell device having one or more insulating layers. The thick-film paste comprises an electrically conductive metal, and a lead-tellurium-oxide dispersed in an organic medium.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/800,592, filed Mar. 13, 2013, which in turn was a continuation ofU.S. application Ser. No. 13/100,533, filed May 4, 2011 and now U.S.Pat. No. 8,497,420, issued Jul. 30, 2013, and further claims benefit ofU.S. Provisional Patent Application Ser. No. 61/331,006, filed May 4,2010; U.S. Provisional Patent Application Ser. No. 61/440,117, filedFeb. 7, 2011; U.S. Provisional Patent Application Ser. No. 61/445,508,filed Feb. 22, 2011; and U.S. Provisional Patent Application Ser. No.61/467,003, filed Mar. 24, 2011. Each of said applications isincorporated herein for all purposes by reference thereto.

FIELD OF THE INVENTION

The present invention provides a thick-film paste for printing thefront-side of a solar cell device having one or more insulating layers.The thick-film paste includes an electrically conductive metal, and alead-tellurium-oxide dispersed in an organic medium.

TECHNICAL BACKGROUND

A conventional solar cell structure with a p-type base has a negativeelectrode that is typically on the front-side (sun-side) of the cell anda positive electrode on the back-side. Radiation of an appropriatewavelength falling on a p-n junction of a semiconductor body serves as asource of external energy to generate hole-electron pair chargecarriers. These electron-hole pair charge carriers migrate in theelectric field generated by the p-n semiconductor junction and arecollected by a conductive grid or metal contact applied to the surfaceof the semiconductor. The current generated flows to the externalcircuit.

Conductive pastes (also termed inks) are typically used to form theconductive grids or metal contacts. Conductive pastes typically includea glass frit, a conductive species (e.g., silver particles), and anorganic medium. To form the metal contacts, conductive pastes areprinted onto a substrate as grid lines or other patterns and then fired,during which electrical contact is made between the grid lines and thesemiconductor substrate.

However, crystalline silicon PV cells are typically coated with ananti-reflective coating such as silicon nitride, titanium oxide orsilicon oxide to promote light adsorption, which increases the cell'sefficiency. Such anti-reflective coatings also act as an insulator,which impairs the flow of electrons from the substrate to the metalcontacts. To overcome this problem, the conductive ink should penetratethe anti-reflective coating during firing to form metal contacts havingelectrical contact with the semiconductor substrate. Formation of astrong bond between the metal contact and the substrate is alsodesirable.

The ability to penetrate the anti-reflective coating and form a strongbond with the substrate upon firing is highly dependent on thecomposition of the conductive ink and firing conditions. Efficiency, akey measure of PV cell performance, is also influenced by the quality ofthe electrical contact made between the fired conductive ink and thesubstrate.

To provide an economical process for manufacturing PV cells with goodefficiency, there is a need for thick-film paste compositions that canbe fired at low temperatures to penetrate an anti-reflective coating andprovide good electrical contact with the semiconductor substrate.

SUMMARY

One aspect of the present invention is a thick-film paste compositionincluding:

-   -   a) 85 to 99.5% by weight of an electrically conductive metal or        derivative thereof, based on total solids in the composition;    -   b) 0.5 to 15% by weight based on solids of a        lead-tellurium-oxide, wherein the mole ratio of lead to        tellurium of the lead-tellurium-oxide is between 5/95 and 95/5;        and    -   c) an organic medium.

Another aspect of the present invention is a process including:

(a) providing a semiconductor substrate including one or more insulatingfilms deposited thereon;

(b) applying a thick-film paste composition onto the one or moreinsulating films to form a layered structure,

wherein the thick-film paste composition includes:

-   -   i) 85 to 99.5% by weight of an electrically conductive metal or        derivative thereof, based on total solids in the composition;    -   ii) 0.5 to 15% by weight based on solids of a        lead-tellurium-oxide, wherein the mole ratio of lead to        tellurium of the lead-tellurium-oxide is between 5/95 and 95/5;        and    -   iii) an organic medium; and        (c) firing the semiconductor substrate, one or more insulating        films, and thick-film paste, forming an electrode in contact        with the one or more insulating layers and in electrical contact        with the semiconductor substrate.

Another aspect of this invention is an article including:

a) a semiconductor substrate;

b) one or more insulating layers on the semiconductor substrate; and

c) an electrode in contact with the one or more insulating layers and inelectrical contact with the semiconductor substrate, the electrodeincluding an electrically conductive metal and lead-tellurium-oxide.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A through 1F depict various steps in a process employed in thefabrication of a semiconductor device in an embodiment of the presentdisclosure.

FIG. 1A depicts a substrate used in the fabrication of a photovoltaicdevice;

FIG. 1B depicts application of an n-type diffusion layer;

FIG. 1C depicts removal of the n-type diffusion layer from most surfacesof the substrate;

FIG. 1D depicts application of an insulating, anti-reflection coating;

FIG. 1E depicts printing of a thick-film paste composition; and

FIG. 1F depicts formation of electrodes of the photovoltaic device.

Reference numerals shown in the various views of FIGS. 1A through 1F areexplained below.

-   10: p-type silicon substrate-   20: n-type diffusion layer-   30: insulating film-   40: p+ layer (back surface field, BSF)-   60: aluminum paste deposited on back-side-   61: aluminum back electrode (obtained by firing back-side aluminum    paste)-   70: silver or silver/aluminum paste deposited on back-side-   71: silver or silver/aluminum back electrode (obtained by firing    back-side silver paste)-   500: thick-film paste deposited on front-side-   501: front electrode (formed by firing the thick-film paste)

DETAILED DESCRIPTION

Solar-powered photovoltaic systems are considered to be environmentallyfriendly in that they reduce the need for fossil fuels.

The present invention provides compositions that can be used tomanufacture photovoltaic devices with improved electrical performance.The thick-film paste composition includes:

-   -   a) 85 to 99.5% by weight of an electrically conductive metal or        derivative thereof, based on total solids in the composition;    -   b) 0.5 to 15% by weight based on solids of a        lead-tellurium-oxide, wherein the mole ratio of lead to        tellurium of the lead-tellurium-oxide is between 5/95 and 95/5;        and    -   c) an organic medium.

As defined herein, the organic medium is not considered to be part ofthe solids including the thick-film paste composition.

Electrically Conductive Metal

The electrically conductive metal is selected from the group consistingof silver, copper and palladium. The electrically conductive metal canbe in a flake form, a spherical form, a granular form, a crystallineform, a powder, or other irregular forms and mixtures thereof. Theelectrically conductive metal can be provided in a colloidal suspension.

When the metal is silver, it can be in the form of silver metal, silverderivatives, or mixtures thereof. Exemplary derivatives include: alloysof silver, silver oxide (Ag₂O), silver salts such as AgCl, AgNO₃,AgOOCCH₃ (silver acetate), AgOOCF₃ (silver trifluoroacetate), or silverorthophosphate, Ag₃PO₄, for example. Other forms of silver compatiblewith the other thick-film paste components can also be used.

In one embodiment, the electrically conductive metal or derivativesthereof is from about 85 to about 99.5 wt % of the solid components ofthe thick-film paste composition. In a further embodiment, theelectrically conductive metal or derivatives thereof is from about 90 toabout 95 wt % of the solid components of the thick-film pastecomposition.

In an embodiment, the solids portion of the thick-film paste compositionincludes about 85 to about 99.5 wt % spherical silver particles. In oneembodiment, the solids portion of the thick-film paste compositionincludes about 85 to about 90 wt % silver particles and about 1 to about9.5 wt % silver flakes.

In one embodiment, the thick-film paste composition includes coatedsilver particles that are electrically conductive. Suitable coatingsinclude phosphate and surfactants. Suitable surfactants includepolyethyleneoxide, polyethyleneglycol, benzotriazole,poly(ethyleneglycol)acetic acid, lauric acid, oleic acid, capric acid,myristic acid, linolic acid, stearic acid, palmitic acid, stearatesalts, palmitate salts, and mixtures thereof. The salt counter-ions canbe ammonium, sodium, potassium and mixtures thereof.

The particle size of the silver is not subject to any particularlimitation. In one embodiment, an average particle size is 0.5-10microns; in another embodiment, the average particle size is 1-5microns. As used herein, “particle size” or “D50” is intended to mean“average particle size”; “average particle size” means the 50% volumedistribution size. Volume distribution size may be determined by LASERdiffraction and dispersion method using a Microtrac particle sizeanalyzer.

Lead-Tellurium-Oxide

The lead-tellurium-oxide (Pb—Te—O) can be prepared by mixing TeO₂ andlead oxide powders, heating the powder mixture in air or anoxygen-containing atmosphere to form a melt, quenching the melt,grinding and ball-milling the quenched material, and screening themilled material to provide a powder with the desired particle size. Thelead oxide powders may include one or more component selected from thegroup consisting of: PbO, Pb₃O₄, and PbO₂. Firing the mixture of leadand tellurium oxides is typically conducted to a peak temperature of 800to 1200° C. The molten mixture can be quenched, for example, on astainless steel platen or between counter-rotating stainless steelrollers to form a thick platelet. The resulting platelet can be milledto form a powder. Typically, the milled powder has a D50 of 0.1 to 3.0microns. In an embodiment, the Pb—Te—O formed in this way may be atleast partially crystalline.

Typically, the mixture of PbO and TeO₂ powders includes 5 to 95 mol % oflead oxide and 5 to 95 mol % of tellurium oxide, based on the combinedpowders. In one embodiment, the mixture of PbO and TeO₂ powders includes30 to 85 mol % of lead oxide and 15 to 70 mol % of tellurium oxide,based on the combined powders. In another embodiment, the mixture of PbOand TeO₂ powders includes 30 to 65 mol % of lead oxide and 35 to 70 mol% of tellurium oxide, based on the combined powders.

In some embodiments, the mixture of PbO and TeO₂ powders furtherincludes one or more other metal compounds. Suitable other metalcompounds include TiO₂, Li₂O, B₂O₃, PbF₂, SiO₂, Na₂O, K₂O, Rb₂O, Cs₂O,Al₂O₃, MgO, CaO, SrO, BaO, V₂O₅, ZrO₂, MoO₃, Mn₂O₃, Ag₂O, ZnO, Ga₂O₃,GeO₂, In₂O₃, SnO₂, Sb₂O₃, Bi₂O₃, BiF₃, P₂O₅, CuO, NiO, Cr₂O₃, Fe₂O₃,CoO, Co₂O₃, and CeO₂. Tables 1 and 2 list some examples of powdermixtures containing PbO, TeO₂ and other optional metal compounds thatcan be used to make lead-tellurium oxides. This list is meant to beillustrative, not limiting. In an embodiment, the weight ratio ofelemental lead to elemental tellurium in the lead-tellurium-oxide iswithin the range of 0.87 to 9.51.

Therefore as used herein, the term “Pb—Te—O” may also include metaloxides that contain oxides of one or more elements selected from thegroup consisting of Si, Sn, Li, Ti, Ag, Na, K, Rb, Cs, Ge, Ga, In, Ni,Zn, Ca, Mg, Sr, Ba, Se, Mo, W, Y, As, La, Nd, Co, Pr, Gd, Sm, Dy, Eu,Ho, Yb, Lu, Bi, Ta, V, Fe, Hf, Cr, Cd, Sb, Bi, F, Zr, Mn, P, Cu, Ce, andNb.

Organic Medium

The inorganic components of the thick-film paste composition are mixedwith an organic medium to form viscous pastes having suitableconsistency and rheology for printing. A wide variety of inert viscousmaterials can be used as the organic medium. The organic medium can beone in which the inorganic components are dispersible with an adequatedegree of stability during manufacturing, shipping and storage of thepastes, as well as on the printing screen during a screen-printingprocess.

Suitable organic media have rheological properties that provide stabledispersion of solids, appropriate viscosity and thixotropy for screenprinting, appropriate wettability of the substrate and the paste solids,a good drying rate, and good firing properties. The organic medium cancontain thickeners, stabilizers, surfactants, and/or other commonadditives. The organic medium can be a solution of polymer(s) insolvent(s). Suitable polymers include ethyl cellulose, ethylhydroxyethylcellulose, wood rosin, mixtures of ethyl cellulose and phenolic resins,polymethacrylates of lower alcohols, and the monobutyl ether of ethyleneglycol monoacetate. Suitable solvents include terpenes such as alpha- orbeta-terpineol or mixtures thereof with other solvents such as kerosene,dibutylphthalate, butyl carbitol, butyl carbitol acetate, hexyleneglycol and alcohols with boiling points above 150° C., and alcoholesters. Other suitable organic medium components include:bis(2-(2-butoxyethoxy)ethyl adipate, dibasic esters such as DBE, DBE-2,DBE-3, DBE-4, DBE-5, DBE-6, DBE-9, and DBE 1B, octyl epoxy tallate,isotetradecanol, and pentaerythritol ester of hydrogenated rosin. Theorganic medium can also include volatile liquids to promote rapidhardening after application of the thick-film paste composition on asubstrate.

The optimal amount of organic medium in the thick-film paste compositionis dependent on the method of applying the paste and the specificorganic medium used. Typically, the thick-film paste compositioncontains 70 to 95 wt % of inorganic components and 5 to 30 wt % oforganic medium.

If the organic medium includes a polymer, the organic composition mayinclude 8 to 15 wt % polymer.

Preparation of the Thick-Film Paste Composition

In one embodiment, the thick-film paste composition can be prepared bymixing the conductive metal powder, the Pb—Te—O powder, and the organicmedium in any order. In some embodiments, the inorganic materials aremixed first, and they are then added to the organic medium. Theviscosity can be adjusted, if needed, by the addition of solvents.Mixing methods that provide high shear may be useful.

Another aspect of the present invention is a process comprising:

a) providing a semiconductor substrate comprising one or more insulatingfilms deposited onto at least one surface of the semiconductorsubstrate;

(b) applying a thick-film paste composition onto at least a portion ofthe one or more insulating films to form a layered structure, whereinthe thick-film paste composition includes:

-   -   i) 85 to 99.5% by weight of an electrically conductive metal or        derivative thereof, based on total solids in the composition;    -   ii) 0.5 to 15% by weight based on solids of a        lead-tellurium-oxide, wherein the mole ratio of lead to        tellurium of the lead-tellurium-oxide is between 5/95 and 95/5;        and    -   iii) an organic medium; and        (c) firing the semiconductor substrate, one or more insulating        films, and thick-film paste forming an electrode in contact with        the one or more insulating layers and in electrical contact with        the semiconductor substrate.

In one embodiment, a semiconductor device is manufactured from anarticle comprising a junction-bearing semiconductor substrate and asilicon nitride insulating film formed on a main surface thereof. Theprocess includes the steps of applying (for example, coating orscreen-printing) onto the insulating film, in a predetermined shape andthickness and at a predetermined position, a thick-film pastecomposition having the ability to penetrate the insulating layer, thenfiring so that thick-film paste composition reacts with the insulatingfilm and penetrates the insulating film, thereby effecting electricalcontact with the silicon substrate.

One embodiment of this process is illustrated in FIGS. 1A through 1F.

FIG. 1A shows a single-crystal silicon or multi-crystalline siliconp-type substrate 10.

In FIG. 1B, an n-type diffusion layer 20 of the reverse polarity isformed to create a p-n junction. The n-type diffusion layer 20 can beformed by thermal diffusion of phosphorus (P) using phosphorusoxychloride (POCl₃) as the phosphorus source. In the absence of anyparticular modifications, the n-type diffusion layer 20 is formed overthe entire surface of the silicon p-type substrate. The depth of thediffusion layer can be varied by controlling the diffusion temperatureand time, and is generally formed in a thickness range of about 0.3 to0.5 microns. The n-type diffusion layer may have a sheet resistivity ofseveral tens of ohms per square.

After protecting one surface of the n-type diffusion layer 20 with aresist or the like, as shown in FIG. 1C, the n-type diffusion layer 20is removed from most surfaces by etching so that it remains only on onemain surface. The resist is then removed using an organic solvent or thelike.

Next, in FIG. 1D, an insulating layer 30 which also functions as anantireflection coating is formed on the n-type diffusion layer 20. Theinsulating layer is commonly silicon nitride, but can also be aSiN_(x):H film (i.e., the insulating film includes hydrogen forpassivation during subsequent firing processing), a titanium oxide film,or a silicon oxide film. A thickness of about 700 to 900 A of a siliconnitride film is suitable for a refractive index of about 1.9 to 2.0.Deposition of the insulating layer 30 can be by sputtering, chemicalvapor deposition or other methods.

Next, electrodes are formed. As shown in FIG. 1E, a thick-film pastecomposition of this invention is screen-printed on the insulating film30, and then dried. In addition, aluminum paste 60 and back-side silverpaste 70 are screen-printed onto the back-side of the substrate, andsuccessively dried. Firing is carried out at a temperature of 750 to850° C. for a period of from several seconds to several tens of minutes.

Consequently, as shown in FIG. 1F, during firing, aluminum diffuses fromthe aluminum paste into the silicon substrate on the back-side, therebyforming a p+layer 40, containing a high concentration of aluminumdopant. This layer is generally called the back surface field (BSF)layer, and helps to improve the energy conversion efficiency of thesolar cell. Firing converts the dried aluminum paste 60 to an aluminumback electrode 61. The back-side silver paste 70 is fired at the sametime, becoming a silver or silver/aluminum back electrode 71. Duringfiring, the boundary between the back-side aluminum and the back-sidesilver assumes the state of an alloy, thereby achieving electricalconnection. Most areas of the back electrode are occupied by thealuminum electrode, owing in part to the need to form a p+layer 40. Atthe same time, because soldering to an aluminum electrode is impossible,the silver or silver/aluminum back electrode is formed on limited areasof the backside as an electrode for interconnecting solar cells by meansof copper ribbon or the like.

On the front-side, the thick-film paste composition 500 of the presentinvention sinters and penetrates through the insulating film 30 duringfiring, and thereby achieves electrical contact with the n-typediffusion layer 20. This type of process is generally called “firethrough.” This fired-through state, i.e., the extent to which the pastemelts and passes through the insulating film 30, depends on the qualityand thickness of the insulating film 30, the composition of the paste,and on the firing conditions. When fired, the paste 500 becomes theelectrode 501, as shown in FIG.

In one embodiment, the insulating film is selected from titanium oxide,aluminum oxide, silicon nitride, SiN_(x):H, silicon oxide, siliconcarbon oxynitride, a silicon nitride film containing carbon, a siliconoxide film containing carbon, and silicon oxide/titanium oxide films.The silicon nitride film can be formed by sputtering, a plasma enhancedchemical vapor deposition (PECVD), or a thermal CVD process. In oneembodiment, the silicon oxide film is formed by thermal oxidation,sputtering, or thermal CFD or plasma CFD. The titanium oxide film can beformed by coating a titanium-containing organic liquid material onto thesemiconductor substrate and firing, or by a thermal CVD.

In this process, the semiconductor substrate can be a single-crystal ormulti-crystalline silicon electrode.

Suitable insulating films include one or more components selected from:aluminum oxide, titanium oxide, silicon nitride, SiN_(x):H, siliconoxide, silicon carbon oxynitride, a silicon nitride film containingcarbon, a silicon oxide film containing carbon, and siliconoxide/titanium oxide. In one embodiment of the invention, the insulatingfilm is an anti-reflection coating (ARC). The insulating film can beapplied to a semiconductor substrate, or it can be naturally forming,such as in the case of silicon oxide.

In one embodiment, the insulating film includes a layer of siliconnitride. The silicon nitride can be deposited by CVD (chemical vapordeposition), PECVD (plasma-enhanced chemical vapor deposition),sputtering, or other methods.

In one embodiment, the silicon nitride of the insulating layer istreated to remove at least a portion of the silicon nitride. Thetreatment can be a chemical treatment. The removal of at least a portionof the silicon nitride may result in an improved electrical contactbetween the conductor of the thick-film paste composition and thesemiconductor substrate. This may result in improved efficiency of thesemiconductor device.

In one embodiment, the silicon nitride of the insulating film is part ofan anti-reflective coating.

The thick-film paste composition can be printed on the insulating filmin a pattern, e.g., bus bars with connecting lines. The printing can beby screen printing, plating, extrusion, inkjet, shaped or multipleprinting, or ribbons.

In this electrode-forming process, the thick-film paste composition isheated to remove the organic medium and sinter the metal powder. Theheating can be carried out in air or an oxygen-containing atmosphere.This step is commonly referred to as “firing.” The firing temperatureprofile is typically set so as to enable the burnout of organic bindermaterials from dried thick-film paste composition, as well as any otherorganic materials present. In one embodiment, the firing temperature is750 to 950° C. The firing can be conducted in a belt furnace using hightransport rates, for example, 100-500 cm/min, with resulting hold-uptimes of 0.05 to 5 minutes. Multiple temperature zones, for example 3-11zones, can be used to control the desired thermal profile.

Upon firing, the electrically conductive metal and Pb—Te—O mixturepenetrate the insulating film. The penetration of the insulating filmresults in an electrical contact between the electrode and thesemiconductor substrate. After firing, an interlayer may be formedbetween the semiconductor substrate and the electrode, wherein theinterlayer includes one or more of tellurium, tellurium compounds, lead,lead compounds, and silicon compounds, where the silicon may originatecomes from the silicon substrate and/or the insulating layer(s). Afterfiring, the electrode includes sintered metal that contacts theunderlying semiconductor substrate and may also contact one or moreinsulating layers.

Another aspect of the present invention is an article formed by aprocess comprising:

(a) providing a semiconductor substrate comprising one or moreinsulating films deposited onto at least one surface of thesemiconductor substrate;

(b) applying a thick-film paste composition onto at least a portion ofthe one or more insulating films to form a layered structure, whereinthe thick-film paste composition includes:

-   -   i) 85 to 99.5% by weight of electrically conductive metal or        derivative thereof, based on total solids in the composition;    -   ii) 0.5 to 15% by weight based on solids of a        lead-tellurium-oxide, wherein the mole ratio of lead to        tellurium of the lead-tellurium-oxide is between 5/95 and 95/5;        and    -   iii) an organic medium, and        (c) firing the semiconductor substrate, one or more insulating        films, and thick-film paste, forming an electrode in contact        with the one or more insulating layers and in electrical contact        with the semiconductor substrate.

Such articles may be useful in the manufacture of photovoltaic devices.In one embodiment, the article is a semiconductor device comprising anelectrode formed from the thick-film paste composition. In oneembodiment, the electrode is a front-side electrode on a silicon solarcell. In one embodiment, the article further includes a back electrode.

EXAMPLES

Illustrative preparations and evaluations of thick-film pastecompositions are described below.

Example I Lead-Tellurium-Oxide Preparation

Lead-Tellurium-Oxide Preparation of Glass Frits of Tables 1 & 2

Mixtures of TeO₂ powder (99+% purity) and PbO powder (ACS reagent grade,99+% purity) and optionally, PbF2, SiO2, B2O3, P2O5, lead phosphate,SnO2, SnO, Li2O, Li2(CO3), Li(NO3), V2O5, Ag2O, Ag2(CO3), Ag(NO3) weretumbled in a polyethylene container for 30 min to mix the startingpowders. The starting powder mixture was placed in a platinum crucibleand heated in air at a heating rate of 10° C./min to 900° C. and thenheld at 900° C. for one hour to melt the mixture. The melt was quenchedfrom 900° C. by removing the platinum crucible from the furnace andpouring the melt onto a stainless steel platen. The resulting materialwas ground in a mortar and pestle to less than 100 mesh. The groundmaterial was then ball-milled in a polyethylene container with zirconiaballs and isopropyl alcohol until the D₅₀ was 0.5-0.7 microns. Theball-milled material was then separated from the milling balls, dried,and run through a 100 mesh screen to provide the flux powders used inthe thick film paste preparations.

TABLE 1 Illustrative examples of powder mixtures that can be used tomake suitable lead-tellurium oxides. Powder Wt % Wt % Wt % Wt % Wt % Wt% Wt % Wt % Wt % mixture PbO TeO₂ PbF₂ SiO₂ B₂O₃ P₂O₅ SnO₂ Li₂O V₂O₅ Wt% Ag₂O A 32.95 67.05 B 38.23 51.26 10.50 C 67.72 32.28 D 72.20 27.80 E80.75 19.25 F 59.69 9.30 16.19 14.82 G 75.86 9.26 14.88 H 48.06 51.550.39 I 48.16 51.65 0.19 J 47.44 50.88 1.68 K 47.85 51.33 0.82 L 41.7644.80 0.32 0.80 12.32 M 46.71 50.10 3.19 N 46.41 49.78 3.80 O 45.1148.39 6.50 P 44.53 47.76 7.71 Q 48.05 51.54 0.41 R 47.85 51.33 0.82 S47.26 50.70 2.04 T 45.82 49.19 4.99 U 48.04 51.53 0.42 V 39.53 28.2632.21

TABLE 2 Glass frit compositions in weight percent TeO2/PbO Glass # (moleratio) PbO TeO2 PbF2 SiO2 Bi2O3 BiF3 SnO2 Ag2O SrO Al2O3 MgO Na2O In2O3BaO WO3 NiO 1 0.17 80.21 9.79 10.00 2 0.22 63.09 9.80 17.12 10.00 3 0.6956.50 27.96 15.54 4 0.69 50.84 25.17 10.01 13.98 5 0.69 53.51 26.50 5.2714.72 6 1.36 48.56 47.12 4.32 7 0.81 49.80 28.76 21.44 8 1.15 45.7037.49 16.81 9 1.11 53.13 42.12 4.75 10 1.50 47.11 50.52 2.37 11 1.5442.92 47.30 8.76 1.02 12 1.58 44.29 49.99 5.72 13 1.86 42.56 56.54 0.8914 1.86 42.15 55.99 1.86 15 1.50 47.37 50.81 1.83 16 1.86 42.96 57.04 171.22 53.36 46.64 18 1.00 50.64 36.21 13.15 19 1.00 52.68 37.67 9.65 201.50 32.87 51.00 16.13 21 0.82 63.09 36.91 22 0.67 67.72 32.28 23 0.5472.20 27.80 24 1.50 47.24 50.68 2.08 25 1.50 44.24 47.47 8.29 26 1.0053.62 38.34 8.05 Note: the compositions in the table are displayed inweight percent, based on the weight of the total glass composition. TheTeO₂/PbO ratios is a molar ratio between only TeO₂ and PbO of thecomposition.Lead-Tellurium-Oxide Preparation of Glass Frits of Tables 3

The lead-tellurium-lithium-oxide (Pb—Te—Li—Ti—O) compositions of Table 3were prepared by mixing and blending Pb₃O₄ and TeO₂ powders, andoptionally, as shown in Table 3, SiO₂, P₂O₅, Pb₂P₂O₇, Ag₂O, Ag(NO₃)and/or SnO₂. The blended powder batch materials were loaded to aplatinum alloy crucible then inserted into a furnace at 900-1000° C.using an air or O₂ containing atmosphere. The duration of the heattreatment was 20 minutes following the attainment of a full solution ofthe constituents. The resulting low viscosity liquid resulting from thefusion of the constituents was then quenched by metal roller. Thequenched glass was then milled, and screened to provide a powder with aD₅₀ of 0.1 to 3.0 microns.

TABLE 3 Frit compositions in weight percent Glass # SiO2 PbO P2O5 Ag2OSnO2 TeO2 27 44.53 7.71 47.76 28 59.22 40.78 29 41.72 13.54 44.75 3080.75 19.25 31 1.66 41.85 0.86 9.58 1.16 44.89 32 58.31 41.69 33 5.9554.27 5.41 1.23 33.14 Note: the compositions in the table are displayedin weight percent, based on the weight of the total glass composition.

Example II Paste Preparation

Thick Film Paste Preparation of Tables 5, 6, 7, & 8

The organic components of the thick-film paste and the relative amountsare given in Table 4.

TABLE 4 Organic components of the thick-film paste Component Wt. %2,2,4-trimethyl-1,3-pentanediol monoisobutyrate 5.57 Ethyl Cellulose(50-52% ethoxyl) 0.14 Ethyl Cellulose (48-50% ethoxyl) 0.04N-tallow-1,3-diaminopropane dioleate 1.00 Hydrogenated castor oil 0.50Pentaerythritol tetraester of perhydroabietic acid 1.25 Dimethyl adipate3.15 Dimethyl glutarate 0.35

The organic components were put into Thinky mixing jar (Thinky USA, Inc)and Thinky-mixed at 2000 RPM for 2 to 4 min until well-blended. Theinorganic components (Pb—Te—O powders and silver conductive powders)were tumble-mixed in a glass jar for 15 min. The total weight of theinorganic components was 88 g, of which 85-87 g was silver powder and1-3 g was the mixture of PbO and TeO₂ powders. One third of inorganiccomponents were then added to the Thinky jar containing the organiccomponents and mixed for 1 min at 2000 RPM. This was repeated until allof the inorganic components were added and mixed. The paste was cooledand the viscosity was adjusted to between 200 and 500 Pas by addingsolvent and then mixing for 1 min at 200 RPM. This step was repeateduntil the desired viscosity was achieved. The paste was then roll-milledat a 1 mil gap for 3 passes at zero psi and 3 passes at 75 psi. Thedegree of dispersion was measured by fineness of grind (FOG). The FOGvalue is typically equal to or less than 20/10 for thick-film pastes.The viscosity of each paste was measured on a Brookfield viscometer witha #14 spindle and a #6 cup. The viscosity of the paste was adjustedafter 24 hrs at RT to between 200 and 320 Pas. Viscosity was measuredafter 3 min at 10 RPM in a viscometer.

TABLE 5 Composition of thick-film pastes Wt % Wt % Wt % Example Mol %TeO₂ in Additive Conductive Organic Number Pb—Te—O Additive in PasteMetal Vehicles 1 60 1.0 87.0 12.0 2 60 1.0 87.0 12.0 3 60 2.0 86.0 12.04 60 2.0 86.0 12.0 5 60 3.0 85.0 12.0 6 60 3.0 85.0 12.0 7 50 1.0 87.012.0 8 50 1.0 87.0 12.0 9 50 2.0 86.0 12.0 10 50 2.0 86.0 12.0 11 50 3.085.0 12.0 12 50 3.0 85.0 12.0Thick Film Paste Preparation of Tables 9, 10, 12, & 13

Paste preparations, in general, were prepared using the followingprocedure: The appropriate amount of solvent, medium and surfactant fromTables 9, 10, 12, & 13 were weighed and mixed in a mixing can for 15minutes.

Since Ag is the major part of the solids, it was added incrementally toensure better wetting. When well mixed, the paste was repeatedly passedthrough a 3-roll mill at progressively increasing pressures from 0 to250 psi. The gap of the rolls was set to 2 mils. The paste viscosity wasmeasured using a Brookfield viscometer and appropriate amounts ofsolvent and resin were added to adjust the paste viscosity toward atarget of between 230 and 280 Pa-sec. The degree of dispersion wasmeasured by fineness of grind (FOG). A typical FOG value for a paste isless than 20 microns for the fourth longest, continuous scratch and lessthan 10 microns for the point at which 50% of the paste is scratched.

To make the final pastes used to generate the data in Tables 9, 10, 12 &13, 2 to 3 wt % of a frit from Table 1 was mixed into a portion of theAg paste and dispersed by shear between rotating glass plates known toone skilled in the art as a muller. To make the final pastes of Tables12 & 13 three separate pastes were made by 1) an appropriate amounts ofAg was added to an appropriate amount of the vehicle of Table 4 wererolled milled, 2) an appropriate amount of the 1^(st) glass frit fromTable 3 was added to appropriate amount of the vehicle of Table 4 wereroll milled, and 3) an appropriate amount of the 2^(nd) glass frit fromTable 3 was added to the appropriate amount of vehicle of Table 4 wasroll milled. Appropriate amounts of the Ag paste and the frit pasteswere mixed together using a planetary centrifugal mixer (ThinkyCorporation, Tokyo, Japan).

Table 11 shows the combined frit composition of the examples of Table 12and 13. The combined frit compositions shown in Table 11 are calculatedusing the frit compositions of Table 3 in the blending ratio of Tables12 & 13.

The paste examples of Table 5, 7, 8, 9, 12, and 13 were made using theprocedure described above for making the paste compositions listed inthe table according to the following details. Tested pastes contained 85to 88% silver powder. These examples used a singular spherical silverwith a D₅₀=2.0 μm.

Example III Solar Cell Preparation

Solar Cell Preparation of the examples in Tables 6, 7, & 8

Solar cells for testing the performance of the thick-film paste weremade from 175 micron thick Q. Cell multi-crystalline silicon wafers witha 65 ohm/sq phosphorous-doped emitter layer which had an acid-etchedtextured surface and 70 nm thick PECVD SiN_(x) antireflective coating.The solar cells were supplied by Q-Cells SE, OT Thalheim, Germany. Thewafers were cut into 28 mm×28 mm wafers using a diamond wafering saw.Wafers were screen-printed after cut-down using an AMI-Presco MSP-485screen printer to provide a bus-bar, eleven conductor lines at a 0.254cm pitch, and a full ground-plane, screen-printed aluminum back-sideconductor. After printing and drying, cells were fired in a BTUInternational rapid thermal processing belt furnace. The firingtemperatures shown in Table 3 were the furnace set-point temperatures,which were approximately 125° C. greater than the actual wafertemperature. The fired conductor line median line width was 120 micronsand mean line height was 15 microns. The median line resistivity was3.0E-6 ohm·cm. Performance of the 28 mm×28 mm cells is expected to beimpacted by edge effects that reduce the overall solar cell fill factor(FF) by ˜5%.

Solar Cell Preparation of the Examples in Tables 9, 10, 12, & 13

Pastes were applied to 1.1″×1.1″ dicing-saw-cut multi-crystallinesilicon solar cells with a phosphorous-doped emitter on a p-type base.The paste from example #1 was applied to a DeutscheCell (DeutscheCell,Germany) multi-crystalline wafer with a 62Ω/□ emitter and pastes fromexamples #2 through #6 were applied to a Gintech (Gintech EnergyCorporation, Taiwan) multi-crystalline wafer with a 55Ω/□ emitter. Thesolar cells used were textured by isotropic acid etching and had ananti-reflection coating (ARC) of SiN_(x):H. Efficiency and fill factor,as shown in Tables 9, 10, 12, & 13, were measured for each sample. Foreach paste, the mean and median values of the efficiency and fill factorfor 5 to 12 samples are shown. Each sample was made by screen-printingusing a ETP model L555 printer set with a squeegee speed of 250 mm/sec.The screen used had a pattern of 11 finger lines with a 100 μm openingand 1 bus bar with a 1.5 mm opening on a 20 μm emulsion in a screen with325 mesh and 23 μm wires. A commercially available Al paste, DuPontPV381, was printed on the non-illuminated (back) side of the device.

The device with the printed patterns on both sides was then dried for 10minutes in a drying oven with a 250° C. peak temperature. The substrateswere then fired sun-side up with a CF7214 Despatch 6 zone IR furnaceusing a 560 cm/min belt speed and 550-600-650-700-800-905 to 945° C.temperature set points. The actual temperature of the part was measuredduring processing. The estimated peak temperature of each part was740-780° C. and each part was above 650° C. for a total time of 4seconds. The fully processed samples were then tested for PV performanceusing a calibrated ST-1000 tester.

Examples IV Solar Cell Performance: Efficiency and Fill Factor

Test Procedure: Efficiency and Fill Factor for the Examples of Tables 6,7, & 8

Solar cell performance was measured using a ST-1000, Telecom STV Co. IVtester at 25° C.+/−1.0° C. The Xe Arc lamp in the IV tester simulatedsunlight with a known intensity, and irradiated the front surface of thecell. The tester used a four-contact method to measure current (I) andvoltage (V) at approximately 400 load resistance settings to determinethe cell's I-V curve. Solar cell efficiency (Eff), fill factor (FF), andseries resistance (Rs) were calculated from the I-V curve. Rs isespecially affected by contact resistivity (ρc), conductor lineresistance and emitter sheet resistance. Since conductor lineresistances and sheet resistances were nominally equivalent for thevarious samples, the differences in Rs were primarily due to ρc.Ideality factor was determined using the Suns-VOC technique. Theideality factor is reported at 0.1 sun irradiance.

Median values for efficiency, fill factor, series resistance, andideality factors for solar cells prepared using the thick-film pastes ofExamples 1-12 were determined and are summarized in Table 6. The meanand median values for efficiency for solar cells prepared using thethink-film paste of examples 13-27 were determined and are summarized inTable 7. The mean and median values for fill factor for solar cellsprepared using the think-film paste of examples 13-27 were determinedand are summarized in Table 8.

TABLE 6 Performance of Pastes Mol % Median Median Median TeO₂ in PeakFiring Effi- Fill Series Median Pb—Te—O Temperature ciency FactorResistance Ideality Ex. Additive (° C.). (%) (%) (ohm{circumflex over( )}cm²) Factor 1 60 930 14.86 76.2 1.50 1.70 2 60 940 15.07 76.9 1.401.50 3 60 930 15.08 77.2 1.42 1.10 4 60 940 15.27 77.6 1.37 1.60 5 60930 14.87 77.5 1.39 1.40 6 60 940 14.92 77.7 1.38 1.10 7 60 930 15.3277.1 1.40 1.70 8 50 940 15.03 76.4 1.44 1.80 9 50 930 14.45 74.4 1.582.20 10 50 940 14.6 74 1.44 2.30 11 50 930 14.29 73.6 1.54 2.40 12 50940 14.26 73.7 1.48 2.10

TABLE 7 Mean and median efficiency (Eff %) of Pastes Glass Exampleamount Eff. (%) Eff. (%) Eff. (%) Eff. (%) Eff. (%) Eff. (%) Eff. (%)Eff. (%) Eff. (%) Number Glass # (wt %) mean median mean median meanmedian mean median mean 13 1 4.5 14.90 14.89 15.00 14.92 15.03 14 2 4.514.13 14.27 14.85 14.79 14.95 15 3 2.5 14.13 14.12 14.32 16 6 2.1 13.8413.96 14.24 17 8 1.5 15.27 15.35 15.34 18 10 3 13.15 12.87 13.56 13.5713.70 19 13 2 12.06 12.45 11.50 12.03 12.54 20 25 2  8.27  7.94 11.9512.29 13.16 21 A 2 14.14 14.34 14.50 14.71 15.50 22 C 1 13.57 14.0317.71 14.92 14.92 23 D 1 13.64 13.92 14.72 14.81 14.93 24 B 2 14.3414.38 14.60 14.52 15.06 145.08 15.00 25 K 2 13.58 13.24 12.35 14.5914.95 26 J 2 13.16 14.04 13.34 15.03 11.61 27 M 2 14.95 14.92 15.0615.14 15.20 15.17 15.35 28 O 2 14.37 14.55 14.73 14.95 15.17 15.27 15.5529 15 2 12.00 30 16 2 12.51 13.33 14.41 14.43 14.47 31 17 2 13.41 13.7714.36 14.47 14.97 32 18 2 14.91 33 19 2 14.88 34 21 2 14.11 14.03 13.7235 22 2 14.50 14.45 14.73 36 23 2 13.22 13.19 12.88 37 20 2 14.90 14.9415.29 Example Eff. (%) Eff. (%) Eff. (%) Eff. (%) Eff. (%) Eff. (%) Eff.(%) Eff. (%) Eff. (%) Eff. (%) Eff. (%) Number median mean median meanmedian mean median mean median mean median 13 15.01 14.34 14.16 13.4313.44 14 14.98 14.92 14.94 15.01 15.08 15 14.23 13.86 13.96 14.19 14.1216 14.35 14.30 14.37 13.77 14.19 17 15.28 15.50 15.49 15.33 15.34 1813.59 12.66 12.93 11.70 11.88 19 12.53 13.10 13.09 13.72 13.65 20 13.1713.52 14.39 13.52 14.39 21 15.00 22 14.83 14.86 14.86 23 14.98 14.8114.84 24 15.19 15.37 15.42 25 14.95 15.14 15.21 26 15.29 15.34 15.41 2715.27 15.19 15.22 28 15.33 15.06 15.27 29 12.30 13.78 13.90 14.38 14.9514.55 14.48 14.43 14.55 30 14.51 14.57 14.55 14.93 14.94 31 14.97 14.8114.89 15.18 15.13 32 14.80 15.17 15.13 15.23 15.33 15.26 15.20 15.0314.93 33 14.91 14.73 14.73 14.69 14.70 15.11 15.02 14.94 14.96 34 13.6814.17 14.16 14.18 13.94 14.27 14.80 35 14.79 14.54 14.64 14.94 14.8614.60 14.66 36 12.80 11.81 11.75 11.38 11.40 11.82 11.83 37 15.25 14.8514.83 15.43 15.46 15.29 15.20

TABLE 8 Mean and median fill factor (FF) of Pastes Glass 900 C. 910 C.915 C. 920 C. 930 C. Example amount FF FF FF FF FF Number Glass # (wt %)FF mean median FF mean median FF mean median FF mean median FF meanmedian 13 1 4.5 76.84 77.60 76.82 76.80 76.82 77.30 14 2 4.5 72.58 73.3075.74 76.10 75.80 75.90 15 3 2.5 74.42 74.70 74.88 74.80 16 6 2.1 71.8472.80 73.44 73.90 17 8 1.5 78.32 78.30 78.86 78.90 18 10 3 68.46 68.4071.56 73.30 71.24 71.30 19 13 2 62.82 64.10 60.44 61.30 64.88 65.60 2025 2 44.24 43.20 60.64 62.00 66.90 67.10 21 A 2 71.96 71.90 72.84 73.6075.96 76.00 22 C 1 70.26 72.10 75.86 76.40 76.48 76.80 23 D 1 72.3073.20 75.26 75.10 76.24 76.70 24 B 2 25 K 2 69.30 69.50 63.86 74.5075.76 76.10 26 J 2 67.70 72.20 66.90 74.60 59.58 75.70 27 M 2 76.3076.10 76.78 77.10 78.02 77.90 77.84 78.10 28 O 2 76.26 76.40 77.30 77.8078.06 77.90 78.04 78.10 29 15 2 65.54 63.00 30 16 2 64.50 69.20 74.4674.90 75.50 75.40 31 17 2 70.23 70.85 74.56 75.20 75.62 75.60 32 18 275.86 76.00 33 19 2 75.66 75.20 34 21 2 75.82 76.10 74.90 76.40 35 22 275.06 75.30 76.03 76.05 36 23 2 70.56 71.20 65.85 65.55 37 20 2 77.5477.40 78.34 78.20 940 C. 945 C. 950 C. 960 C. 970 C. Example FF FF FF FFFF Number FF mean median FF mean median FF mean median FF mean median FFmean median 13 72.66 72.50 69.12 69.10 14 76.52 76.30 77.04 77.20 1572.78 73.10 73.34 73.30 16 73.54 73.10 71.54 73.60 17 79.10 79.00 78.8678.90 18 65.96 66.50 60.62 60.00 19 67.68 68.70 71.42 69.10 20 69.3673.10 69.36 68.10 21 22 76.94 77.10 23 75.70 76.10 24 25 76.82 76.70 2676.80 76.70 27 77.52 77.90 28 77.92 77.80 29 69.18 69.80 72.88 74.6074.20 74.50 73.43 73.45 30 74.20 74.20 76.70 77.30 31 75.82 75.60 76.8277.70 32 76.66 77.10 76.64 77.50 77.50 77.75 77.10 77.45 33 75.82 75.8076.70 76.60 76.65 76.50 76.50 76.60 34 76.90 76.85 77.10 77.40 76.5076.40 35 75.85 76.10 75.70 75.90 75.34 76.30 36 63.78 63.45 60.58 60.0061.80 62.30 37 78.26 78.20 78.90 79.10 78.58 78.80Test Procedure: Efficiency and Fill Factor for the Examples of Tables 9,10, 12, & 13

The solar cells built according to the method described herein weretested for conversion efficiency. An exemplary method of testingefficiency is provided below.

In an embodiment, the solar cells built according to the methoddescribed herein were placed in a commercial I-V tester for measuringefficiencies (Telecom STV, model ST-1000). The Xe Arc lamp in the I-Vtester simulated the sunlight with a known intensity, AM 1.5, andirradiated the front surface of the cell. The tester used a multi-pointcontact method to measure current (I) and voltage (V) at approximately400 load resistance settings to determine the cell's I-V curve. Bothfill factor (FF) and efficiency (Eff) were calculated from the I-Vcurve.

TABLE 9 Paste efficiency (Eff %) data for the results of pastes usingselect frits of table 3 Efficiency (Eff %) Frit level 905 C. 915 C. 920C. 925 C. 940 C. 945 C. Example # Glass # (wt %) Mean Median Mean MedianMean Median Mean Median Mean Median Mean Median 38 29 2 14.65 14.66 3930 3.35 13.08 13.07 13.05 13.33 12.91 12.84 40 31 3.45 13.56 13.55 13.3113.45 14.10 13.99 41 33 2 13.18 13.14 12.33 12.41

TABLE 10 Paste fill factor (FF) data for the results of pastes usingselect frits of table 3 Fill Factor (FF) Frit level 905 C. 915 C. 920 C.925 C. 940 C. 945 C. Example # Glass # (wt %) Mean Median Mean MedianMean Median Mean Median Mean Median Mean Median 38 29 2 70.99 70.83 3930 3.35 69.79 69.04 70.51 71.40 68.84 68.61 40 31 3.45 72.03 72.30 71.0171.38 74.20 74.75 41 33 2 66.33 65.55 62.84 62.35

TABLE 11 Combined frit composition resulting from the blended fritexperiments of Tables 12 and 13 using the frits from Table 3. BlendedGlass Composition Number PbO Ag2O TeO2 I 33.40 20.58 46.02 II 37.7020.92 41.37 III 29.15 29.61 41.24 IV 43.73 14.80 41.47

TABLE 12 The paste efficiency (Eff %) results of paste using a blend oftwo different frits from table 3 (combined frit compositions given inTable 11). Blended Glass 1st frit 2nd frit total frit Efficiency (Eff %)1st 2nd Composition level level level 905 C. 915 C. 925 C. Example #Glass # Glass # Number (wt %) (wt %) (wt %) Mean Median Mean Median MeanMedian 42 27 28 I 1.5 0.5 2 15.31 15.37 14.92 15.13 15.10 15.16 43 32 28II 1.93 1.07 3 14.69 14.76 14.65 14.69 14.80 14.83 44 32 28 III 1.5 1.53 14.77 14.83 15.19 15.32 15.03 15.15 45 32 28 IV 1.6 0.6 2.2 15.3515.29 15.21 15.16 15.13 15.16

TABLE 13 The paste fill factor (FF) results of paste using a blend oftwo different frits from table 3 (combined frit compositions given inTable 11). Blended Glass 1st frit 2nd frit total frit Fill Factor (FF)1st 2nd Composition level level level 905 C. 915 C. 925 C. Example #Glass # Glass # Number (wt %) (wt %) (wt %) Mean Median Mean Median MeanMedian 42 27 28 I 1.5 0.5 2 75.36 75.30 74.83 75.45 75.98 76.37 43 32 28II 1.93 1.07 3 74.20 75.03 73.52 73.57 74.09 75.03 44 32 28 III 1.5 1.53 73.66 74.13 75.23 75.58 74.94 75.03 45 32 28 IV 1.6 0.6 2.2 76.2376.60 76.37 76.17 76.59 76.63

Comparative Example I: Bismuth-Tellurium-Oxide

Preparation of Bismuth-Tellurium-Oxide

A bismuth-tellurium-oxide (Bi—Te—O) containing composition as shown intable 14 was prepared using boron oxide (B₂O₃), zinc oxide (ZnO),titanium oxide (TiO₂), bismuth oxide (Bi₂O₃), tellurium oxide (TeO₂),lithium carbonate (LiCO₃), and lithium phosphate (LiPO₄) and by theprocedure described above in Example I: Lead-tellurium-oxide preparationof glass frits of Table 3.

TABLE 14 Bismuth-tellurium-oxide composition in weight percent of theoxides Glass A (wt %) B₂O₃ 2.09 ZnO 0.98 TiO₂ 0.48 Bi₂O₃ 26.64 TeO₂67.22 P₂O₅ 0.43 Li₂O 2.16 Note: the composition in the table aredisplayed as weight percent, based on the weight of the total glasscompositionPaste Preparation

Pastes using glass A were made by the following procedure. A paste wasmade by mixing the appropriate amount of organic vehicle (Table 4) andAg powder. The Ag paste was passed through a 3-roll mill atprogressively increasing pressures from 0 to 75 psi. The Ag pasteviscosity was measured using a Brookfield viscometer and appropriateamounts of solvent and resin were added to adjust the paste viscositytoward a target of between 230 and 280 Pa-sec. Another paste was made bymixing the appropriate amount of organic vehicle (Table 4) and glasspowder A. The frit paste was passed through a 3-roll mill atprogressively increasing pressures from 0 to 250 psi. The degree ofdispersion of each paste was measured by fineness of grind (FOG). Atypical FOG value for a paste is less than 20 microns for the fourthlongest, continuous scratch and less than 10 microns for the point atwhich 50% of the paste is scratched.

Ag and frit pastes were mixed with a mixed together using a planetarycentrifugal mixer (Thinky Corporation, Tokyo, Japan) to make the finalpaste recipes displayed in table 15.

Solar Cell Preparation and Efficiency and Fill Factor Measurement

Pastes were applied to 1.1″×1.1″ dicing-saw-cut multi-crystallinesilicon solar cells with a phosphorous-doped emitter on a p-type base.The pastes were applied to a DeutscheCell (DeutscheCell, Germany)multi-crystalline wafer with a 62Ω/□ emitter. The solar cells used weretextured by isotropic acid etching and had an anti-reflection coating(ARC) of SiN_(x):H. Efficiency and fill factor, as shown in Table 15,were measured for each sample. Each sample was made by screen-printingusing a ETP model L555 printer set with a squeegee speed of 200 mm/sec.The screen used had a pattern of 11 finger lines with a 100 μm openingand 1 bus bar with a 1.5 mm opening on a 20 μm emulsion in a screen with325 mesh and 23 μm wires. A commercially available Al paste, DuPontPV381, was printed on the non-illuminated (back) side of the device.

The device with the printed patterns on both sides was then dried for 10minutes in a drying oven with a 250° C. peak temperature. The substrateswere then fired sun-side up with a CF7214 Despatch 6 zone IR furnaceusing a 560 cm/min belt speed and 500-550-610-700-800-HZ6 temperatureset points, where HZ6=885, 900 & 915° C. The actual temperature of thepart was measured during processing. The estimated peak temperature ofeach part was 745-775° C. and each part was above 650° C. for a totaltime of 4 seconds. The fully processed samples were then tested for PVperformance using a calibrated ST-1000 tester.

Efficiency and fill factor, shown in Table 15, were measured for eachsample. For each paste, the mean and median values of the efficiency andfill factor for 6 samples are shown.

TABLE 15 Recipe and electrical performance for pastes containing thebismuth-tellurium-oxide glass A of Table 14 Efficiency (Eff %) FillFactor (FF) Ag Frit 900 915 930 900 915 930 Example # Glass # wt % wt %Mean Median Mean Median Mean Median Mean Median Mean Median Mean MedianA A 88.6 2.1 10.04 10.13 12.46 13.01 12.61 12.98 52.8 53.5 65.1 67.364.2 65.6 A A 86.3 4.2 1.14 0.83 1.45 1.42 2.93 2.79 28.7 29.7 29.4 29.530.7 30.1

What is claimed is:
 1. A thick-film paste composition for use in formingan electrical connection in a photovoltaic device comprising asemiconductor substrate having at least one insulating layer on a mainsurface thereof, the composition comprising: a) 95.5 to 99.5% by weightof an electrically conductive metal or derivative thereof, based ontotal solids in the composition; b) 0.5 to 4.5% by weight based onsolids of a lead-tellurium-oxide, wherein the mole ratio of lead totellurium in the lead-tellurium-oxide is between 5/95 and 95/5, thelead-tellurium oxide comprises at most 67.72% of PbO by weight of thelead-tellurium-oxide and comprises at least 32.28% of TeO₂ by weight ofthe lead-tellurium-oxide, and the lead-tellurium-oxide comprises one ormore of P₂O₅, Al₂O₃, TiO₂, Cr₂O₃, Mn₂O₃, Fe₂O₃, CoO, Co₂O₃, NiO, CuO,Ga₂O₃, GeO₂, Rb₂O, Cs₂O, SrO, BaO, In₂O₃, SnO₂, Sb₂O₃, and CeO₂ in anamount ranging from greater than 0 to less than 9% by weight of thelead-tellurium oxide, and the lead-tellurium oxide is at least partiallycrystalline; and c) an organic medium; and wherein the thick-film paste,when fired, is capable of penetrating the at least one insulating layer.2. The thick-film paste composition of claim 1, wherein thelead-tellurium-oxide comprises at least one oxide of one or moreelements selected from the group consisting of Si, Zn, As, Se, Mo, Ag,Nb, Cd, Y, La, Nd, Pr, Gd, Sm, Dy, Eu, Ho, Yb, Lu, Hf, Ta, W, and Bi. 3.The thick-film paste composition of claim 1, comprising thelead-tellurium-oxide in an amount ranging from 0.5 to 3.45% by weightbased on solids.
 4. The thick-film paste composition of claim 1, whereinthe lead-tellurium-oxide comprises at most 63.09% of PbO by weight ofthe lead-tellurium oxide.
 5. The thick-film paste composition of claim1, wherein the lead-tellurium-oxide comprises at least 36.91% of TeO₂ byweight of the lead-tellurium oxide.
 6. The thick-film paste compositionof claim 5, wherein the lead-tellurium-oxide comprises at most 63.09% ofPbO by weight of the lead-tellurium oxide.
 7. The thick-film pastecomposition of claim 1, wherein the lead-tellurium-oxide is Cd-free. 8.The thick-film paste composition of claim 1, wherein thelead-tellurium-oxide is As-free.
 9. The thick-film composition of claim1, wherein the lead-tellurium-oxide further comprises Li₂O in an amountranging from greater than 0 to 5% by weight of the lead-tellurium oxide.10. The thick-film composition of claim 1, wherein thelead-tellurium-oxide further comprises Ag₂O in an amount ranging fromgreater than 0 to 29.61% by weight of the lead-tellurium oxide.
 11. Thethick-film composition of claim 1, wherein the lead-tellurium-oxidecomprises one or more glass frits.
 12. The thick-film composition ofclaim 1, wherein the electrically conductive metal comprises silver.